1. Field of the Invention
The invention relates to an apparatus for providing a panning effect in a raster scan video display system, and specifically for implementing a toroidal pan in an image memory.
2. Description of the Prior Art
Graphic display terminals are increasingly used for display of computer generated pictorial information. By displaying such information on a cathode ray tube (CRT) display screen, the user can view the pictorial image essentially as soon as it is generated by the host computer. This is particularly advantageous when performing computer assisted design work, since changes in a design can be displayed pictorially as soon as they are made. By providing zoom and pan capabilities, the operator is aided in the use of the video display system. Panning is the shifting of the displayed portion of an image to adjacent portions of the image. This feature allows the operator of the video display system to smoothly move the "display window" provided by the CRT across a larger image which itself could not be completely displayed at one time on the CRT.
CRT based graphic display terminal systems fall into two fundamental classes: raster scan and vector generated displays. In a vector generated CRT graphic display terminal, the image is decomposed into a list of lines which are individually drawn by the electron beam of the CRT during each image refresh on the CRT. As the drawing increases in complexity, thereby increasing the number of lines to be drawn, the rate at which the image can be drawn decreases due to the limitation in the number of lines which may be drawn by the system in a given time.
A raster scan CRT graphic display terminal system, such as disclosed in U.S. Pat. No. 4,070,710 to J. Sukonick, et al., works on a principle similar to that of the household television. The face of the CRT screen is painted by the CRT's electron beam in a series of horizontal lines. The intensity of the electron beam is modulated as the CRT screen is scanned to provide varying intensities of light or color on the screen, thereby forming the desired image. In contrast to the vector scan system, a raster scan system for displaying images on a CRT provides a unvarying image generation load since the raster scan is a repetitive operation, repeated usually about 60 times per second, which is unaffected by the complexity of the image being displayed.
Raster scan graphic display systems typically contain an image memory in which digitized information as to the image to be displayed is stored. One or more memory cells in the image memory contains information concerning color, intensity, position, and other parameters for an element of the image. The smallest element for which information is stored is commonly called a pixel (picture element). A raster readout apparatus is usually provided to coordinate the fetching of pixel information from the image memory with the raster scan and to reformat the information into a composite video signal for use in controlling the CRT.
Use of such an image memory allows for the generation of special effects on the CRT. For instance, a background grid for the image is easily provided by associated circuitry. Additionally, a zoom feature, i.e., magnification of an image, can be created by using information concerning one pixel for several adjacent positions on the raster scan. An additional effect possible on such systems is that of panning, i.e., apparent movement of the image being displayed across a larger image.
Typically, graphic display terminals use a 12 inch or 15 inch CRT tube to present the image being displayed. Inasmuch as such devices may be used to display full size blueprints of many inches in width and length, the CRT operator is at a disadvantage in that he cannot simultaneously inspect or view the entire image, i.e., blueprint, on the CRT based system. The cost of providing a CRT with a display surface the same size as that of a blueprint is exorbitantly expensive or even technologically impossible at this time. Panning is an operator aid which compensates for the disadvantages caused by the small CRT screen. Even though an image of 40 inches or 50 inches in length or width can not be simultaneously displayed on the CRT screen, panning allows the operator to move the display window provided by the CRT as he desires from one portion of the image to another. This facility, in conjunction with the multiple advantages a CRT based graphic display terminal system offers, such as real time modification of images, have been fundamental to the success of such systems.
Typically, the operator controls the panning effect via a keyboard into which panning commands may be entered, or a "joy stick". A joy stick is a lever which can be moved by the operator in two dimensions and is used to convey positional or directional information to the graphic display system in a convenient manner.
There are several methods used in the prior art for providing a panning effect. One method is to rewrite the entire image memory between consecutive raster scans. The information rewritten into the image memory would form a slightly displaced image on the CRT, giving the effect of a slight pan of the image. The rewriting would continue between consecutive raster scans, giving the desired panning effect. The major disadvantage of this technique is that it is technologically difficult to rewrite the entire display image in the image memory between raster scans, about 1 msec. High speed image memory and complex digital logic are required to rewrite the image memory in the short CRT vertical retrace period of time between raster scans.
An alternative scheme used in the prior art is that based on double buffering the image memory. Two image memories, each able to store information sufficient for an entire raster scan, are provided. When a pan command is received, the image data for the image to be used by the next raster scan is written into the alternate image memory. During the raster scan from the alternate image memory, the first image memory may be suitably updated with an additionally displaced image. This alternation of image memories for each raster scan continues until the desired destination of the panning command is reached. This scheme also has several disadvantages. It, of course, requires twice as much image memory to be provided in the video display system. Additionally, the circuitry must be designed so that it may operate at a rate sufficient to rewrite data on an image into the appropriate image memory within the time consumed by one raster scan, e.g., 17 msec or 1/60 of a second. Although not nearly as demanding as the first mentioned scheme, high speed memory and digital logic circuitry must be used in order to provide the capability of rewriting an entire image memory within the allotted time.
A third method used in the prior art for providing a panning effect in raster scan video display systems is to provide image memory sufficient to store a larger portion of the image than which can be simultaneously displayed. When a panning command is received, the information is read out in raster fashion from a different part of the image memory. Such a scheme has the advantage that within the limits of the image memory, panning can be as fast as desired inasmuch as the data is already available in the image memory. Additionally, the circuitry used to rewrite the image memory need not be of as high speed a design as in the earlier methods inasmuch as the image memory rewriting may be unrelated to the raster scan. The principle disadvantage to such a scheme is that substantial additional image memory is required. Although panning is possible, it is limited to the supplied image memory. Where it is desired to pan across a large portion of a image, the memory necessary to store the entire image is substantial. Accordingly, such a scheme is useful when only a limited panning is acceptable.
It is an objective of the herein disclosed invention to provide a panning apparatus for use in a raster scan video display system which does not require as high a speed of rewriting circuitry as prior art methods have required, yet which is useable for panning across an arbitrarily large image without requiring substantial additional image memory. It is a further objective of the herein disclosed invention to provide such a panning apparatus which is also able to provide a flicker free panning effect without degraded image quality. It is a further objective of this invention to provide a panning effect which is smooth and has operator specifiable direction and rate.